Japanese Unexamined Patent Application Publication (JP-A) No. 2010-159005 and JP-A No. 2010-69927 describe pipe expanding of 6xxx series aluminum alloy hollow extrusions by electromagnetic forming. JP-A No. 2007-254833 and JP-A No. 2005-105327 describe 6xxx series aluminum alloy hollow extrusions having excellent expanding workability and being formed by electromagnetic forming. JP-A No. 2010-196089 describes a 6xxx series aluminum alloy hollow extrusion having excellent expanding workability and being formed by hydroforming. The 6xxx series aluminum alloy hollow extrusions described in these patent literatures each subjected to pipe expanding under a condition of T1 temper where the material exhibits high formability, and thereafter aged.
Independently, the application of pipe expanding to 7xxx series aluminum alloy hollow extrusions has been examined, which materials contain alloy elements such as Zn, Mg, and Cu in large amounts and thereby have higher strengths after temper aging than other series alloys do. The 7xxx series aluminum alloy extrusions, however, even being materials (T1 temper materials) after press quenching and before temper aging, undergo hardening due to natural aging and suffer from inferior formability. For this reason, the 7xxx series aluminum alloy hollow extrusions, when subjected to pipe expanding at a practical-level expansion rate of 5% or more, readily stiffer from cracking in the expanded (worked) sites. This tendency is more remarkable in higher alloys.
For better formability, a reversion treatment has been performed on 7xxx series aluminum alloys hardened by natural aging, as described typically in JP-A No. H07-305151, JP-A No. H10-168553, JP-A No. 2005-194020, and JP-A No. 2007-119853.